It is known in the art of optical compensation that the phase retardation of light varies according to wavelength, causing color shift and contrast ratio reduction. This wavelength dependence (or dispersion) characteristic of the compensation film may be taken into account when designing an optical device so that color shift is reduced and contrast ratio increased. Wavelength dispersion curves are defined as “normal (or proper)” or “reversed” with respect to the compensation film having positive and negative retardance (or retardation). A compensation film with positive retardance (positive A- or C-plate) may have a normal dispersion curve in which the value of phase retardation is increasingly positive toward shorter wavelengths or a reversed dispersion curve in which the value of phase retardation is decreasingly positive toward shorter wavelengths. A compensation film with negative retardance (negative A- or C-plate) may have a normal dispersion curve in which the value of phase retardation is increasingly negative toward shorter wavelengths or a reversed dispersion curve in which the value of phase retardation is decreasingly negative toward shorter wavelengths. Exemplary shapes of these curves are depicted in FIG. 1.
Wave plates are customarily named as follows in accordance with their refractive index profiles:
positive A-plate: nx>ny=nz; negative A-plate: nx<ny=nz; positive C-plate: nx=ny<nz; negative C-plate: nx=ny>nz, wherein, nx and ny represent in-plane refractive indices, and nz is the thickness refractive index.
The above wave plates are uniaxial birefringent plates. A wave plate can also be biaxial birefringent, where nx, ny, and nz all have different values; it is customarily referred to as a biaxial film.
An A-plate is a wave plate commonly used as a retarder in an optical device. It is a birefringent material capable of manipulating the polarization state or phase of the light beam traveling through the medium. The A-plate optical retarder has a refractive index profile of nx>ny=nz, wherein nx and ny represent in-plane refractive indices and nz represents the thickness-direction refractive index. Such a wave plate exhibits a positive in-plane retardation (Re) as expressed by Re=(nx−ny)×d, wherein d is the thickness of the wave plate. Re is also often denoted as Ro.
An A-plate having in-plane retardation (Re) equal to a quarter of a light wavelength (λ), Re=λ/4, is called quarter wave plate (QWP). A quarter wave plate is capable of converting an incident linearly polarized light into circularly polarized light. Thus, a quarter wave plate is commonly used in combination with a linear polarizer to provide a circular polarizer in an optical device. Circularly polarized light has been used in polarized three-dimensional (3D) display systems to produce stereoscopic image projection. Circular polarization has an advantage over linear polarization in that viewers are able to tilt their heads and move around naturally without seeing distorted 3D images. Such 3D display systems require viewers to wear glasses, commonly referred to as 3D glasses, equipped with circular polarizing films in order to see 3D images. Recently, there has been much increased interest in 3D consumer products such as TVs and computer displays. Thus, there is a demand for improved 3D glasses with circular polarizing films. Specifically, there is a need for a quarter wave plate having normal wavelength dispersion, which has been found to have the utility for 3D glasses to improve the viewing quality. It is known that such quarter wave plates can be achieved by using polycarbonate or cyclic polyolefin. However, in a device based on such quarter wave plates, a cellulose ester film is required to protect the polyvinyl alcohol based polarizer. It would be advantageous if the quarter wave plate is based on cellulose ester film and can also function as a protective film for the polarizer. Accordingly, this invention is further directed to quarter wave plates based on cellulose ester.
In order to have a normal wavelength dispersion curve, the in-plane retardation (Re) of a quarter wave plate should satisfy the following equations:Re(450)/Re(550)>1 and Re(650)/Re(550)<1wherein Re(450), Re(550), and Re(650) are in-plane retardations at the light wavelengths of 450 nm, 550 nm, and 650 nm respectively.
The positive A-plate, however, also exhibits a negative out-of-plane retardation Rth, which is defined as Rth=[nz−(nx+ny)/2]×d with a value of |Re/2| arising from its orientation. The term “|Re/2|” means the absolute value of Re/2. This characteristic can be beneficial when a negative Rth is desirable in an optical device. For example, in a vertically aligned (VA) mode liquid crystal display (LCD), the liquid crystal molecules in the LC cell are aligned in a homeotropic manner, which results in positive out-of-plane retardation. A wave plate with a negative Rth, thus, can provide an out-of-plane compensation in addition to in-plane compensation in VA-LCD. In other types of devices, such as in-plane switch (IPS) mode LCD and 3D glasses, however, the Rth exhibited in the A-plate is not desirable since it can give rise to phase shift in off-axis light and lead to light leakage. Thus, there exists a further need in the art to provide a quarter wave plate having reduced out-of-plane retardation for improved viewing quality.
It is well-known that optical films made from cellulose esters, cyclolefins, polycarbonates, polyimides, acrylics and polyesters are widely used in flat panel displays, such as LCDs and OLEDs. Among them, cellulose esters such as cellulose triacetate (CTA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB), are playing important role for the liquid crystal display (LCD) industry. Most notable is their use as protective and viewing angle compensation films used in conjunction with polarizer sheets. These films are typically made by solvent casting, and then are laminated to either side of an oriented, iodinated polyvinyl alcohol (PVA) polarizing film to protect the PVA layer with regards to scratching and moisture ingress, while also increasing structural rigidity. Cellulose esters have many performance advantages over other materials such as acrylics, cyclolefins, polycarbonates, polyimides, PET, etc. However optical birefringence requirements currently often dictate how compensation films will be used.
Besides serving a protective role, these compensation films are also extremely important for improving the contrast ratio, wide viewing angle and color shift performance of the LCD. For example, for a typical set of crossed polarizers used in an LCD, there is significant light leakage along the diagonals (leading to poor contrast ratio), particularly as the viewing angle is increased. It is known that various combinations of optical films can be used to correct or “compensate” for this light leakage. These films must have certain well defined birefringences (or retardations) that vary depending on the type of liquid crystal cell used, since the liquid crystal cell itself will also impart an undesirable optical retardation that must be corrected. Some of these compensation films are easier to make than others, so compromises are often made between performance and cost. Also, while most of the compensation and protective films are made by solvent casting, there is a push to make more films by melt extrusion. Key optical parameters will now be defined.
Compensation films are commonly quantified in terms of birefringence which is, in turn, related to the refractive index “n”. The refractive index is typically in the range of 1.4 to 1.8 for polymers in general, and approximately 1.46 to 1.50 for cellulose esters. The higher the refractive index, the slower the speed the light wave propagates through that given material.
For a non-oriented isotropic material, the refractive index will be the same regardless of the polarization state of the entering light wave. As the material becomes oriented, or otherwise anisotropic, the refractive index becomes dependent on material direction. For purposes of the present invention, there are three refractive indices of importance denoted nx, ny and nz corresponding to the film plane x- and y-direction, the film thickness z-direction, respectively. As the material becomes more anisotropic (e.g. by stretching it), the difference between any two refractive indices will increase. This difference is referred to as the “birefringence.” Because there are many combinations of material directions to choose from, there are correspondingly different values of birefringence. The two that are the most common, namely in-plane birefringence An, and the out-of-plane birefringence Δnth , are defined asΔne=nx−ny   (1a)Δnth=nz−(nx+ny)/2   (1b)
The birefringence Δne is a measure of the relative in-plane orientation between the x- and y-directions and is dimensionless. Generally, x-direction is chosen to be the larger stretching direction in comparison with y-direction. In contrast Δnth gives a measure of the orientation of the thickness direction, relative to the average planar orientation.
Another term often used with regards to optical films is the optical retardation R. R is simply the birefringence times the thickness d of the film in question. Thus,Re=Δned=(nx−ny)d   (2a)Rth=Δnthd=[nz−(nx+ny)/2]d   (2b)
Retardation is a direct measure of the relative phase shift between the two orthogonal optical waves and is typically reported in units of nanometers (nm). Note that the definition of Rth varies with some authors particularly with regards to the sign (+/−).
Materials are also known to vary with regards to their birefringence or retardation behavior. For example, most materials when stretched will exhibit a higher refractive index along the stretch direction and a lower refractive index perpendicular to the stretch. This follows because, on a molecular level, the refractive index is typically higher along the polymer chain's axis and lower perpendicular to the chain. These materials are commonly termed “positively birefringent” and represent most standard polymers including almost all commercial cellulose esters.
Another useful parameter termed the “intrinsic birefringence” is a material property and quantifies the birefringence that would occur if the material was fully stretched with all chains perfectly aligned in one direction.
There are two other much rarer classes of material, namely “negative birefringent” and “zero birefringent”. Negative birefringent polymers exhibit a higher refractive index perpendicular to the stretch direction, and consequently also have a negative intrinsic birefringence. Certain styrenics and acrylics are known to have negative birefringent behavior due to their rather bulky side groups. Zero birefringence, in contrast, is a special case and represents materials that show no birefringence with stretching and thus have a zero intrinsic birefringence. Such materials are ideal for optical applications as they can be molded, stretched, or otherwise stressed during processing without showing any optical retardation or distortion. Such materials are also extremely rare.
The actual compensation film(s) that is used in an LCD can take on a variety of forms including biaxial films where all three refractive indices differ and two optical axes exist, and uniaxial films having only one optical axis where two of the three refractive indices are the same. The important point is that the type of compensation film that can be made is limited by the birefringence characteristics of the polymer (i.e. positive or negative). A few examples will now be discussed.
In the case of uniaxial films, a film having refractive indices such thatnx>ny=nz “+A” plate   (3a)is denoted as a “+A” plate. In these films, the x direction of the film has a high refractive index whereas the y and thickness directions are approximately equal in magnitude (and lower than nx). This type of film is also referred to as a positive uniaxial structure with the optical axis along the x-direction. Such films are easy to make by uniaxially stretching a positively birefringent material, using for example, a film drafter.
In contrast, a “−A” uniaxial film is defined asnx<ny=nz “−A” plate   (3b)where the x-axis refractive index is lower than the other directions (which are approximately equal). The most common method for making a −A plate is to stretch a negative birefringent polymer.
Another class of uniaxial optical film is the C plate which can also be “+C” or “−C”. The difference between a C and A plate is that in the former, the unique refractive index (or optical axis) is in the thickness direction as opposed to in the plane of the film. Thus,nz>ny=nx “+C” plate   (4a)nz<ny=nx “−C” plate   (4b)
C-plates can be made by biaxial stretching if the relative stretch in the x and y directions is held constant. Alternately they can be made by compression forming. Compressing or equibiaxially stretching an initially isotropic, positive intrinsic birefringent material will result in a −C plate since the effective orientation direction is in the plane of the film. Conversely, a +C plate is made by compressing or equibiaxially stretching an initially isotropic film made with negative intrinsic birefringent material.
A third and more common option for producing C-plates takes advantages of the stresses that form during solvent casting of a film. Tensile stresses are created in the plane of the film due to the restraint imposed by the casting substrate or casting belt, which are also equi-biaxial in nature. These tend to align the chains in the plane of the film resulting in −C or +C films for positive and negative intrinsic birefringent materials respectively. As most cellulose ester films used in displays are solvent cast, and all are essentially positive birefringent, then it is apparent that solvent cast cellulose esters normally only produce −C plates. These films can also be uniaxially stretched to produce +A plates.